M-1 Mic Preamp M-2 Mic Preamp M-1 Personal Mic


M-1 Mic Preamp M-2 Mic Preamp M-1 Personal Mic
1728 Brummel St.
Evanston, IL 60202
Phone: 847-864-8060
Toll-free: 866-379-1450
Fax: 847-864-8076
M-1 Mic Preamp
M-2 Mic Preamp
M-1 Personal Mic Preamp
June 1, 2011
The M-1, M-2 and M-1 Personal mic preamps are among the finest mic preamps in the world. They provide
the accuracy and transparency that is missing in other mic preamps. No matter what your application, they
will provide superior results. The world's best input transformer (Jensen JT-16-B), best op-amp (990C
discrete class-A op-amp) and the elimination of all coupling capacitors from the signal path combine to
provide the highest performance.
The M-1 and M-2 have a 19” wide rack-mount chassis (1.75”H x 19”W x 8”D) and can be ordered with one, two, three or four
channels. The M-1 Personal mic preamp has an 8” wide chassis with a capacity of one channel. There are two meter options,
and the best Jensen output transformer (JT-11-BMQ) is available as an option. These options and basic channels can be easily
added later.
The M-1 was introduced in 1987. The M-2, introduced in 1996, is a variation of the M-1. The first difference is the gain
controls: The M-1 has a two-section gain-pot providing continuously variable adjustment in two overlapping ranges of 12 to 40
dB and 32 to 60 dB (higher gain available on request). The “HIGH GAIN” switch changes ranges. The M-2 has a 16-position
gain-switch with 1% metal-film resistors, providing accurate and repeatable gain-settings from 15 to 60 dB in steps of 3 dB.
Further differences involve the push-button switch to the right of the gain-pot or gain-switch. The M-1 has a “HIGH GAIN”
switch as mentioned above. Since this switch is not required in the M-2, the p.c. board layout was modified to allow that switch
to be used in the M-2 as either a “20Ω MIC” switch, or a “20dB PAD” switch, depending on how the board is assembled. The
20Ω MIC switch provides optimum matching of microphones with extremely low output impedances. The 20dB PAD switch
attenuates the input signal by 20 dB prior to the JT-16-B input transformer, providing a maximum input level of +29 dBu.
The Jensen JT-16-B Input Transformer is
Jensen's best mic-input model. Jensen is
known worldwide for their superior audio
transformers. If you thought transformers
were a compromise, you haven't heard the
The JT-16-B is a large, low impedance
ratio (150:600Ω) transformer made with a
proprietary 80% nickel (nickel-iron-mo­
lybdenum) core material. The large size
allows it to handle extremely high signal
levels of +12dBu at 30Hz and above,
+9.7dBu at 20Hz. The low ratio provides
less distortion, flatter frequency response
and more linear phase response than more
(150:15kΩ). The proprietary 80% nickel
core material is far superior to, and much
more expensive than the steel often found
in other transformers.
The JT-16-B outperforms transformerless
mic preamps because it eliminates the
input coupling capacitors that are usually
required with transformerless designs.
Capacitors degrade the audio signal
because they have a property known as
dielectric absorption. Some of the signal
passing through the capacitor is absorbed
by the dielectric of the capacitor, then
released a short time later. This smears
the signal. Transformerless designs
require these capacitors to keep the
phantom supply voltage from reaching
the circuitry of the preamp. Transformers
eliminating the need for the capacitors.
The JT-16-B provides better common
mode rejection than transformerless
designs, important in electrically noisy
environments. It handles common mode
voltages as high as ±300V peak.
Transformerless designs are usually
limited to maximum voltages equal to
their power supply voltages, typically
±15V to ±18V.
The 990C Discrete Op-Amp is faster,
quieter, more powerful and better
sounding than the typical monolithic opamps found in other equipment. Each
individual (discrete) transistor, resistor,
diode, capacitor and inductor of the
990C has been carefully chosen for its
task. This provides a level of
performance that is not possible in a
monolithic op-amp where all components
are fabricated on the same tiny chip of
silicon. The 990C operates from ±24V
power supplies, allowing output levels of
greater than +24dBu. It can drive long
cables and loads as low as 75Ω, something
monolithic op-amps cannot do. See the 990
data package for further information.
Elimination of All Coupling Capacitors from
the Signal Path results in less degradation
of the audio signal. Two superior
techniques are used to accomplish this:
1. Input bias current compensation
circuitry nulls out the small DC currents
(thus voltages) that flow from the inputs of
the 990C (or any op-amp), voltages that
could cause noise when operating the gain
controls. This circuitry also reduces the
DC offset voltage at the output of the
990C. Most other mic preamps use
coupling capacitors to block the input bias
currents, resulting in signal degradation.
2. DC servo circuitry nulls out the DC
offset voltage at the output of the 990C,
eliminating the need for a traditional
output coupling capacitor to block that
voltage. The signal degradation caused by
that capacitor is also eliminated. See the
schematic on page 7 for details.
Standard Features
M-1: Dual Range Gain Control and “HIGH
GAIN” Switch. The overall gain adjustment
range is 12 to 60 dB, a span of 48 dB.
Rather than cover the entire 48 dB in one
revolution of the gain pot, there are two
smaller, overlapping ranges selected by the
“HIGH GAIN” switch. The low gain
range provides adjustment of 12 to 40 dB,
the high gain range 32 to 60 dB (higher
gain can be provided on request). This
provides 28 dB of adjustment per range,
with 8 dB of overlap. The relatively small
28 dB of adjustment per range provides
great feel and resolution, with sufficient
overlap to keep you “in range” at all times.
There are two reasons for the great feel:
First, with fewer dB of adjustment per
revolution, it is easier to adjust the pot to
the desired gain setting. The second reason
is a more technical one, and very
important. A single 10kΩ pot covering the
entire 48 dB range in a single revolution
could get a bit “touchy” as it approaches
the maximum gain point (minimum
resistance), due to contact resistance
variation (CRV). The gain pot of the M-1
is actually a two section pot having a 10kΩ
section and a 500Ω section. In high gain
applications where CRV would be a
problem with a 10kΩ pot, the 500Ω
section of the pot is used. CRV is
reduced by a factor of twenty, virtually
eliminating any touchiness. See page 5
for details.
The unusually low minimum gain of 12
dB allows the M-1 to handle input levels
as high as +12 dBu before the output is
driven past its +24 dBu maximum output
level. The JT-16-B input transformer can
handle input levels of +12 dBu at 30 Hz
and above, and +8 dBu at 20 Hz.
If you need infinite resolution so you can
set the mic preamp to any gain, or be
able to “ride gain” during a performance,
the gain controls of the M-1 are ideal.
M-2: 16-Position Rotary Gain-Switch with
1% metal-film resistors provides accurate
and resettable gain control from 15 to 60
dB in steps of 3 dB (higher gain can be
provided on request). For situations
where exact gain-matching of channels
or exact resettability is required, the gain
controls of the M-2 are ideal.
M-2: “20Ω MIC” Switch provides optimum
matching with microphones that have a
20Ω output impedance. Note that the
Jensen JT-16-B mic-input transformer
does exceptionally well with just about
any low-impedance microphone, including
those with a 20Ω output impedance, but
this switch provides further refinement.
The switch takes the place of the “HIGH
GAIN” switch of the M-1.
M-2: “20dB PAD” Switch provides a 20dB
resistive pad before the input transformer.
This increases the maximum input level to
+29 dBu for situations where excessively
high input levels are encountered. Most
applications won't need a pad, but for
those that do, this option provides it. This
switch takes the place of the “HIGH
GAIN” switch of the M-1.
Note that you can get an M-2 channel built
with the “20Ω MIC” option or the “20dB
PAD” option, but not both. These options
take advantage of the fact that the “HIGH
GAIN” switch of the M-1 is not needed in
the M-2, and the p.c. board was modified
to allow that switch to be used in one of
three ways.
M-1 and M-2 Common Features:
Polarity Reverse Switch (“POL REV”)
reverses the signal polarity immediately
before the input transformer.
48V ON/OFF Switch (“+48V”) for phantom
power. The phantom supply has more than
enough current to handle any condenser
All Front Panel Switches are LED
Illuminated. A custom clear plastic push
button was developed for the M-1 and M2. Each button's function is marked on the
front surface and is illuminated dimly
when off, brightly when on, each button
with its own LED color. The HIGH GAIN
switch of the M-1 (20Ω MIC or 20dB
PAD for the M-2) uses a red LED, the
POL REV switch uses an amber LED and
the +48V switch uses a green LED. No
guessing about these switches!
Gold Plated XLRs for maximum reliability.
Gold does not tarnish or oxidize.
Ground Lift Switch on Each Channel allows
disconnection of the shield (pin 1) of the
output XLR. This can be helpful in
eliminating ground loops. This minitoggle switch is on the rear panel.
Toroidal Power Transformer with
eliminates hum problems caused by stray
magnetic fields. Some manufacturers use
a separate power supply chassis and
umbilical cord to keep the power
transformer's stray magnetic fields from
interfering with the audio circuitry. In
the M-1 and M-2, the stray magnetic
fields are controlled at the transformer.
Each transformer is carefully tested for
stray magnetic fields under worst-case
full-load conditions. The optimum
rotational position is determined, then
the silicon-iron shielding is added to
assure hum-free performance. Thanks to
these extra details, the transformer can
be built into the M-1 or M-2 chassis. No
separate chassis and umbilical cord to
deal with.
Toroidal power transformers inherently
have lower stray magnetic fields than
conventional EI-core transformers. They
are also smaller and lighter. They are also
much more expensive!
Universal Power Supply. An internal switch
provides six primary voltage choices: 100,
120, 140, 200, 220 and 240 volts. The
power cord is detachable, with a line filter
included in the input connector. These
features allow the preamps to be easily
adapted for use anywhere in the world.
The supply accommodates over/under
voltage situations easily.
Chassis Ground Isolation Switch allows the
chassis ground to be isolated from the
signal ground, or tied to it. This can be
helpful in eliminating ground loops in
certain situations. This mini-toggle switch
is on the rear panel.
Built to Order, the way YOU want it. Start
with only one basic channel if you wish.
Additional channels, meters and output
transformers can be easily field-installed.
The mainframe is ready for all four
channels, with blank panels provided for
unused channels. Have it your way!
VU-1 Meter Card is a very accurate and
informative meter that directly monitors the
output level of the MPC-1 mic preamp card.
There is no need to monitor the input level
of the MPC-1 because the output will clip
before the input transformer saturates.
The VU-1 provides a 20 segment LED
bargraph display and separate “peak” LED
(labeled “PK” on the front panel) to indicate
extremely high signal levels. An LED-illu­
minated front panel switch (green LED)
gives a choice of “Peak” or “VU” meter
ballistics. The “Peak” ballistic provides a
fast attack time for the bargraph so that
transients are fully indicated. The “VU”
ballistic provides a slower attack time,
similar to a standard mechanical VU meter.
The meter scale accurately covers -28 to
+10dB in linear steps of 2dB (15 yellow
LEDs, 5 red LEDs). Easy calibration of the
meter's 0VU operating level is accomplished
by moving an internal plug-in jumper to one
of four positions: 0dBu, +4dBu, +8dBu or
ADJustable. The “ADJ” position covers a
range of -10 to +12 dBu via a 25-turn trim
pot. Standard setting is “+4” (0VU on the
meter scale equals a +4 dBu output level).
The firing point of the separate peak (PK)
LED is calibrated via a 25-turn trim pot for
output levels of 0 to +22dBu. The standard
setting of +22dBu provides at least 2dB of
warning prior to clipping. Jumpers are
provided to choose
BAR mode
(cumulative LEDs) or DOT mode (one
LED at a time) for the display.
Circuitry includes a full-wave rectifier,
peak detector, Peak and VU ballistics, and
a temperature compensated log/linear
converter. The circuitry is DC coupled and
uses high-speed, precision op-amps with
extremely low DC offset voltage and drift
(better op-amps than you find in the signal
path of many consoles!). All of these
features guarantee accurate performance
over a wide temperature range, and for
years to come. On-card voltage regulation
for the op-amp power supplies, and
isolated grounding for the 5 volt LED
power supply assure that the VU-1 will not
interfere with the mic preamp circuitry.
PK-1 Meter Card provides a peak LED
function only. It uses the same full-wave
rectifier, peak detector, 25-turn trim pot for
setting the firing point of the LED, on-card
voltage regulation and isolated grounding
for the 5 volt LED power supply that is
used on the VU-1 card.
JT-11-BMQ Output Transformer is the best
compliments the outstanding line driving
capability of the 990C by providing a
balanced, floating, isolated output. Ground
loop problems are eliminated because the
signal is coupled magnetically rather than
directly, something that transformerless
circuits cannot do. Your application may not
require an output transformer, but if you
need one, the JT-11-BMQ is the best.
PIN 2 or PIN 3 HIGH? There are two polarity
standards in use today for XLR connectors.
The official IEC, SMPTE and AES
standards state that pin 2 is high (relative to
pin 3), while the unofficial standard states
that pin 3 is high (relative to pin 2). The M-1
and M-2 make it very easy to deal with this.
A pair of plug-in jumpers is located next to
each XLR, allowing you to quickly change
from “PIN 2 HIGH” to “PIN 3 HIGH”, or
It is very important to verify the polarity of
the equipment that will be used with the mic
preamp, and to maintain correct polarity
when connecting the mic preamp. Possible
problems range from an audible change due
to an inadvertent reversal of polarity, to
slight degradation of the signal if a
transformer-coupled output is driving an
unbalanced input of the opposite polarity, to
possible damage to the 990C in a directoutput configuration driving an unbalanced
input of the opposite polarity (driving a
short-circuit!). Please specify PIN 2 HIGH
or PIN 3 HIGH!
A few Important Details
Factory Selection of Critical Parts. R2 and
R3, the 6.81kΩ resistors in the phantom
supply network, are matched to 0.1%
tolerance for the best performance.
1% 100ppm Metal Film Resistors are used
instead of the more common 5% 200ppm
carbon film resistors. They provide greater
initial accuracy, better long term stability,
and higher stability at extremes of
Polycarbonate Capacitors are used in
critical timing circuits instead of cheaper
mylar or polyester capacitors. They are
much more stable, and have a more linear
Electrolytic Capacitors with a 105°C
Temperature rating are used instead of the
more common 85°C rated parts. This
higher temperature capability means that
they will last much longer than the lower
rated parts. They will also have better,
more linear performance over a wider
temperature range. Electrolytic capacitors
are more failure prone than most other
components (a good thing to remember
when troubleshooting older equipment).
Sometimes they allow small amounts of
DC current to pass through (leakage
current), causing pots and switches to be
noisy when operated. (NOTE: in the M-1
and M-2 there are no capacitors in the
signal path, so this problem cannot exist).
Other capacitors will short-circuit, or lose
most of their capacitance. Whatever the
failure mode, you have a problem, even in
equipment that never approaches an
operating temperature of 85°C. But not
with these mic preamps!
XLR Connectors are Soldered Directly to
the P.C. Card, minimizing the number of
interconnections for better reliability and
better sound quality.
Fully Sealed Potentiometer and Trim Pots
for long, trouble-free life.
Central Point Grounding and Power
Distribution. Rather than use a
“motherboard”, wiring harnesses are
used to deliver power supplies and
grounds to each channel individually.
This provides the least interaction
between channels.
The KNOB. A knob is a basic device that
should provide three basic things:
1. Good VISUAL indication of setting.
2. Good TACTILE indication of setting.
3. Good TRACTION for your fingers.
Most knobs don't meet all three of the
requirements. Some don't meet any of the
requirements! In addition to these basic
requirements, a knob should look good
and feel good.
Plastic knobs look like . . . well . . .
plastic knobs! Plain round knobs don't
give any tactile indication of which way
they are pointing. Knobs with pointers or
bars sticking out do tell by feel which
way they point, but the protrusion is
often so big that it gets in the way. Some
knobs have an indicator line on top, but
the typical decorative metal finish causes
light reflections from the top like spokes of
a wheel. Which is the indicator line and
which are the spokes?
This knob was developed to meet all
requirements. It is machined out of solid
aluminum, with a nonreflective black
anodized finish. A laser-cut white ceramic
insert is added to create visual and tactile
indication of the knob's setting. The insert
appears as an indicator line on the top of
the knob, and protrudes just enough
(.025”) beyond the side of the knob so that
you can feel it, yet it doesn't get in the way.
Traction is provided by a fine diamond
knurl with sharp, fully formed teeth. The
diamond knurl provides traction for rotary
motion, and for vertical motion to keep
your fingers from slipping “up” and off of
the knob. Straight knurls can only provide
rotary traction. Also, there is a certain
amount of tradition in a diamond knurl.
The knob looks great, feels great, and
works great!
An Extruded Aluminum Chassis was
developed for the front, rear and sides of
the M-1 and M-2 chassis. It solves a
number of packaging problems, providing
a neater, stronger and more efficient
package. The brushed and black anodized
finish looks great, and provides optimum
thermal emission properties. Rack-handles
are provided for easy installation and
handling. Stainless steel threaded inserts
are used for long life and the ability to
Condensed Typical Specifications (0dBu = 0.775V)
E.I.N., 20-20kHz unweighted,
input level, M-1
input level, M-2
input level, M-2 with pad
output level at 990 output
Deviation from linear phase
THD, JT-16-B, (below saturation)
THD, JT-16-B & 990:
60dB gain, 10kΩ load, +24dBu output
40dB gain, 600Ω load, +24dBu output
40dB gain, 75Ω load, +24dBu output
DC offset
150Ω source:
0Ω source:
75Ω load:
Contact Resistance and Contact Resistance Variation
The most familiar specifications for
potentiometers are: resistance value,
tolerance and taper. Even these simple specs
will vary with temperature, time, applied
voltage, number of rotations, etc., but they
are pretty straight forward. However, contact
resistance (CR), and contact resistance
variation (CRV), are specifications that are
unfamiliar to many people.
A pot has a moving contact, or “wiper”, and
it is positioned along the surface of a
understanding the problems of CR and CRV
is to realize that the resistance element has a
small but measurable thickness to it, and the
current flow is not always occurring exactly
at the surface of the element. This is because
the current flow follows the path of least
resistance created by the imperfect blend of
conductive and non-conductive materials
used to make the element. There is a
measurable amount of distance, therefore
resistance, between the contact and the
nearest point of current flow. This contact
resistance (CR) can be as much as 2% of the
pot's resistance value, and will vary as the
contact is moved from one position to the
next (CRV). If the current is flowing near
the surface of the element at the contact
position, CR is low. If the current is flowing
far below the surface at the contact position,
CR is high. For example, if the pot measured
exactly 10kΩ from end to end, and if you
could find the exact electrical midpoint of
the resistive element, it could measure
5200Ω from either end to the contact sitting
at the midpoint. It takes 5000Ω to get to
the midpoint of the resistive element, and
an additional 200Ω to get to the surface of
the element where the contact is, assuming
a worst-case CR of 200Ω.
When a pot is used as a rheostat, as it is in
the gain control of the MPC-1 mic preamp
card, the CR must be added to the basic
element resistance when making gain
calculations. The gain pot of the MPC-1
card is configured so that a reduction of
resistance causes an increase of gain.
For the following example let's assume a
single section 10kΩ pot is used, covering
the entire 48dB adjustment range (12 to
60dB) in a single revolution. Due to the
logarithmic nature of audio, it takes almost
a 2kΩ reduction of resistance (10kΩ to
8kΩ) to provide a 1 dB increase of gain
when going from 12 to 13 dB of gain,
while it takes a mere 10Ω reduction of
resistance (70 to 60Ω) to provide a 1 dB
increase when going from 47 to 48 dB of
gain. The worst-case CR of 200Ω (2% of a
10kΩ pot) would be insignificant in the
first instance (resistance change from
10kΩ to 8kΩ for a gain change of 12 to 13
dB), since a CR of 200Ω is small
compared to the 2kΩ change in gain pot
resistance. But if you were increasing the
gain from 47 to 48 dB by changing the
gain pot setting from 70 to 60Ω, a CR of
200Ω could definitely be a problem,
compared to the desired 10Ω change in
gain pot resistance. Imagine a worst-case
situation: at the theoretical 70Ω position,
contact resistance might happen to add 10Ω,
while at the theoretical 60Ω position, contact
resistance might happen to add 200Ω.
Instead of going from 70Ω to 60Ω, you
would actually be going from 80Ω to 260Ω.
The resistance goes up instead of down, and
the gain is decreased by more than 8 dB
instead of the 1dB increase you planned on!
The next nudge of the control could have
just the opposite effect. It is highly unlikely
that CRV would be that bad, but it is
unpredictable and undesirable.
The gain pot of the MPC-1 mic preamp card
is actually a two section pot (RV1A and
RV1B on the schematic on page 7), with a
10kΩ section and a 500Ω section. In the
low-gain mode (12 to 40 dB), the HIGH
GAIN switch shorts across the 500Ω section
of the pot, leaving just the 10kΩ section
active. The CR of a 10kΩ pot is not a
problem at lower gains, as mentioned earlier.
In the high-gain mode (32 to 60 dB), the
HIGH GAIN switch shorts across the 10kΩ
section, leaving just the 500Ω section active.
Contact resistance would be a problem at
these higher gains if you were still using the
10kΩ pot, but the worst-case CR is just 10Ω
with the 500Ω pot, compared to 200Ω with
the 10kΩ pot. This reduces the CRV
problem by a factor of twenty. The result is a
much smoother, more consistent and higher
resolution gain control.